DSL and ADSL systems use a technique called discrete multitone (DMT) for transmitting data. With DMT, a frequency band up to 1.2 MHz is split up into 256 tones (also referred to as subcarriers or subchannels) each spaced 4.3125 kHz apart. In a DSL/ADSL application, the tones are allocated for use depending on the direction of communication between a central office (CO) and a remote terminal (RT) or customer premises equipment (CPE).
Communication from a CO to a RT/CPE (such as an end user's PC modem) is referred to as “downstream.” The direction of communication from a RT/CPE to the CO is called “upstream.” A higher and wider frequency range, for example, 176 kHz to 1.1 MHz, is allocated to the downstream communication, and a lower frequency range, for example, 10 kHz to 138 kHz, is allocated to the upstream communication.
FIG. 1 schematically illustrates a conventional ADSL transceiver system. A transmit signal (TX) is typically coming from a digital signal processing (DSP) processor 1 through a digital to analog converter (DAC) 2 to an analog front end (AFE) 3. The AFE is a circuit block that provides the interface between the ADSL transceiver and the DSP processor, and typically includes a filter. In order to comply with strict ADSL transmission mask specifications, sufficient filtering must be provided in the transmit direction. The transmit signal is then supplied with sufficient voltage and current by the line driver 5, and coupled via a transformer 7 to a transmission line 9, such as a telephone line or twisted-pair loop. The transmission line 9 has a certain line impedance Z (typically 100 Ω).
As shown in FIG. 1, back termination resistors 6 are inserted between outputs of the line driver 5 and the primary of the transformer 7 in order to properly terminate a signal received from the transmission line 9 (receive signal:RX). That is, the back-termination resistors 6 have a specific resistance RBT so as to match the output impedance Zout of the transceiver and the transmission line impedance Z. When the transformer 7 has a turns ratio of 1:n, the standard value of the back-termination resistors RBT is   Z      2    ⁢          n      2      for a differential line driver as shown in FIG. 1.
Although the back-termination resistors are necessary to prevent undesirable reflection of the receive signal, they waste one-half of the power provided by the line driver amplifiers. Thus, in DSL systems one conventional way of reducing system power is to reduce the value of the back-termination resistors from its standard value. The reduced termination resistance reduces the drop across the resistors and thus increases the proportion of the transmit signal that reaches to the transmission line, allowing the use of a lower supply voltage for the line driver. However, simply reducing a termination resistance causes mistermination of the receive signal as well as reducing the amount of the receive signal developed across these resistors to be sensed by a receive circuit.
An approach termed “active termination” provides a positive feedback from the line driver outputs so as to boost the reduced value of termination resistor and make the effective (or synthesized) output impedance match the line impedance. FIG. 2A illustrates a conventional line driver 10 having a differential amplifier 12 with an active termination architecture. An input signal voltage Vin is input through input resistors R1, and amplified by the differential amplifier 12 to an amplified voltage             V      c        =                                        R            f                                R            1                          ·                              (                          k              +              1                        )                                k            +            1            -                                          R                f                                            R                F                                                        ⁢              V        in              ,where Rf is a feedback resistance of the differential amplifier 12, and RF is a resistance of the positive feedback for the active termination.
As shown in FIG. 2A, the termination resistance Rt has a value reduced from its standard value by factor k, i.e.,             Z              2        ⁢                  n          2                      ⁢    k    ,where k<1. When the line impedance has a typical value of 100 Ω, the back-termination resistance Rt is       50          n      2        ⁢      k    .  It should be noted that for a differential structure, the total termination resistance       Z          n      2        ⁢  kis divided into a pair of termination resistors. Each amplifier output is coupled via a feedback resistance RF to the opposite amplifier input so as to make a positive feedback.
FIG. 2B shows a single-ended structure 10′ corresponding to the line driver 10, for simplicity. As is understood from FIG. 2B, when the value of the feedback resistance RF is chosen to satisfy             R      F        =                  R        f                    1        -        k              ,a synthesized impedance Z′ seen looking into the circuit at the output node is       100          n      2        ,matching the effective output impedance Zout to the line impedance Z=100.
There is a conventional technique to build a second order low pass filter around an amplifier, by adding a relatively small number of extra components. For example, a Rauch configuration is typically chosen because of its robustness against components variations. FIG. 3 illustrates a conventional line driver 14 including a differential amplifier 16 with a Rauch filter configuration. The Rauch filter/amplifier includes an operational amplifier 16, first and second input resistance R1 and R2, a feedback resistance R3, and capacitances ½C1 and C2, as shown in FIG. 3.
As is well understood by those of ordinary skill in the art, the transfer function of the Rauch configuration shown in FIG. 3 is given as follows, which represents the second order filter characteristic:                     V        c                    V        in              ⁢          (      s      )        =                              R          3                          R          1                    ·              1                              R            1                    ⁢                      R            2                    ⁢                      C            1                    ⁢                      C            2                                              s        2            +              s        ·                              G            p                                C            1                              +              1                              R            1                    ⁢                      R            2                    ⁢                      C            1                    ⁢                      C            2                              where             G      p        ≡                  1                  R          1                    +              1                  R          2                    +              1                  R          3                      ,and s is the Laplace variable.
When applied to an ADSL line driver, depending on the sampling rate used in the ADSL system, the Rauch filter can be the only one present or part of a higher order filter. In ADSL applications this filter can be designed to have a cut-off frequency of 138 KHz for the CPE side transceiver, and 1.1 MHz for the CO side transceiver.
However, as shown in FIG. 3, the conventional line driver 14 with the Rauch configuration does not employ an active termination architecture. Also, the conventional impedance synthesis is used only in the line drivers configured as a pure gain stage without any filter characteristics. Because the Rauch filter is inherently frequency dependent, it is unknown to those of ordinary skill in the art how any additional components affect the required filter characteristic, or whether such additional components operate as intended.
As described above, implementing an active termination or impedance synthesis is desirable to reduce the required power of the line driver. It is also desirable to build a low pass filter around a line driver because it can eliminate extra filtering either on-chip or off-chip, so as to reduce the system cost. In addition, it is easier and less expensive to build a low pass filter around the line driver than implementing one in the AFE portion. Furthermore, providing the low-pass filter at the last stage of the transmit signal (i.e., at the line driver amplifier) is more effective in cutting off higher frequency noises. Accordingly, it would be desirable to provide both low-pass filtering and active impedance synthesis in ADSL line drivers to satisfy the transmit mask requirement.